Yes, definitely, given that your arrival isn't too badly delayed. You can easily make it to the city centre by eight using AY's own bus going directly to the main railway station, smack middle of the city (it's a bit more expensive than the public bus but conveniently faster so worth the 5 EUR price). Since you should have you boarding pass for OSL and midday isn't a particularly busy time you could hang around until 10.30 or so and take the same bus back. Taxis are faster but will be around 40 EUR each way, rather pricey.

This gives you two and half hours to walk around the centre, if the weather is nice (May often can be) go to the market square at the harbor and have a breakfast outside, the experience is great.

Alternatively if is raining you could head to the Jumbo shopping mall only a couple of kilometers from the airport. Features all the usual retail thingies, a number of restaurants and a water park/spa/cinema complex which is great for washing away any weariness from the travel - it is quite affordable during the regular workdays, more expensive over the weekend and weekday nights... Jumbo is a short 10 minute bus ride or a 5 minute around 10 EUR taxi ride.

Search this forum for a number of earlier suggestions on the same topic (should we get a sticky?).